The principal function of the thyroid gland is to produce the hormones thyroxine (tetraiodothyronine, T4) and tri-iodothyronine (T3), both of which play essential roles in regulating intermediary metabolism in virtually all tissues and in maturation of the nervous system, skeletal muscle and lungs in the developing fetus and the newborn (Werner and Ingbar, The Thyroid: A fundamental and clinical text (Braverman and Utiger, eds.) (1991) pp. 1-1362, Lippincott, Philadelphia; DeGroot, Endocrinology (DeGroot, ed.) (1995) Grune and Stratton, Orlando, Fla.). Thyroxine and T3 are unique hormones in that both contain iodine as an essential constituent.
The hormone-producing thyroid follicular cells or thyrocytes display a highly specialized ability to transport iodide, the anionic form of iodine. This ability is an apparent cellular adaptation to sequester environmentally scarce iodine, thus ensuring adequate thyroid hormone production in most cases. Nevertheless, insufficient dietary supply of iodine is still prevalent among millions of people in many regions of the world, leading to endemic iodine deficiency disorders (IDD) often associated with lower-than-normal thyroid hormone production (Medeiros-Neto, et al., Thyroid Research, (Robbins and Braverman, eds.), (1976) p. 497, Excerpta Medica, Amsterdam). The translocation of iodide into the thyroid for thyroid hormogenesis involves two separate processes: iodide accumulation and iodide efflux.
Iodide accumulation is the translocation of iodide from the interstitium into the follicular cells across the basolateral plasma membrane. Iodide accumulation is a Na+-dependent active transport process catalyzed by the sodium/iodide symporter, an intrinsic plasma membrane protein located in the basolateral end of thyrocytes that couples the energy released by the inward “downhill” translocation of Na+ down its electrochemical gradient to driving the simultaneous inward “uphill” translocation of iodide against its electrochemical gradient (Carrasco, Biochim. Biophys. Acta. 1154:65-82 (1993)). The Na+ gradient acting as a driving force for iodide accumulation is generated by the ouabain-sensitive, K+out-activated Na+/K+ ATPase. Thus, Na+-dependent iodide accumulation (i.e. sodium/iodide symport activity) is the first and rate-limiting step in the biosynthesis of thyroid hormones. Sodium/iodide symport activity in the thyroid is characteristically blocked by the competitive inhibitor perchlorate.
Iodide efflux is the transfer of iodide from the cytoplasm of thyrocytes towards the colloid across the apical plasma membrane. Iodide efflux is a passive diffusion mechanism that has been proposed to be mediated by an iodide channel located in the apical membrane of thyrocytes (Nilsson, et al., Acta Endocrinol 126:67-74 (1992); Golstein, et al., Am. J. Physiol. 263:C590-C5975 (1992)). The colloid, where the large hormone precursor thyroglobulin (Tg) is stored, is located in the follicular lumen, an extracellular compartment. Iodide is ultimately required at the cell/colloid interface because this is the site where, to a large extent, hormone biosynthesis takes place (Werner and Ingbar, supra; Degroot, supra). Accumulated iodide that has reached the cell/colloid interface is oxidized and incorporated into some tyrosyl residues within the Tg molecule in a reaction catalyzed by thyroid peroxidase (TPO), leading to the subsequent coupling of iodotyrosine residues. This incorporation of iodide into organic molecules is called “iodide organification”, a reaction pharmacologically blocked by such anti-thyroid agents as 6-n-propyl-2-thiouracil (PTU). All steps in the thyroid hormone biosynthetic pathway are stimulated by thyroid stimulating hormone (TSH) secreted from the pituitary. The effect of TSH results from binding of the hormone to the TSH receptor, which is also located in the basolateral membrane of the follicular cells.
Sodium/iodide symporter confers to the thyroid gland its most readily distinctive functional attribute, i.e. its ability to actively accumulate iodine. Sodium/iodide symporter provides the molecular basis for the thyroidal radioiodide uptake test and for thyroid scintigraphy, two thyroid function tests of considerable value as diagnostic aids in a variety of thyroid pathological conditions (Werner and Ingbar, supra; Degroot, supra). For example, the possible existence of thyroid cancer must be ruled out whenever a thyroid nodule is detected. Thyroid nodules that are determined by scintigraphy to accumulate iodine equally or more efficiently than the normal surrounding tissue are generally benign, while most thyroid cancers display markedly reduced iodine accumulation activity relative to healthy tissue. Still, sodium/iodide symporter is sufficiently active in some thyroid cancers and metastases to render them amenable to treatment with radioiodine (Werner and Ingbar, supra; Degroot, supra). Conversely, large doses of radiation reaching the gland via sodium/iodide symporter in the form of iodine isotopes can cause thyroid cancer. The most dramatic example of this is the alarming rise in the incidence of thyroid cancer cases in Ukraine and Belarus in the wake of the 1986 Chernobyl power plant accident (Likhtarev, et al., Nature 375:365 (1995)). In this instance, 131I in the nuclear fallout was ingested largely through milk, mostly by young children, and concentrated in the thyroid via sodium/iodide symporter. Among major thyroid proteins involved in hormogenesis, the TSH receptor, Tg and TPO have all been characterized in considerable molecular detail (Parmentier, et al., Science 246:1620-1622 (1989); Mercken, et al., Nature 316:647-651 (1985); Magnusson, et al., J. Biol. Chem. 262:13885-13888 (1987)). Prior to the present invention however, the cDNA encoding sodium/iodide symporter has not been cloned or characterized.